The present invention relates to a correction of an air-fuel ratio detection value to be used when an air-fuel ratio of an internal combustion engine is feedback controlled, in particular, to a technique for maintaining detection accuracy of the air-fuel ratio to a high level.
Conventionally, there has been known an air-fuel ratio feedback control in which an air-fuel ratio of an engine intake air-fuel mixture is indirectly detected by detecting an oxygen concentration within an engine exhaust by an oxygen sensor, to feedback control a fuel supply quantity so that the air-fuel ratio detected by the oxygen sensor approaches a target air-fuel ratio (refer to Japanese Unexamined Patent Publication No. 60-240840).
In such an air-fuel ratio feedback control, a control in which a target air-fuel ratio is set to a theoretical air-fuel ratio by using the oxygen sensor capable of detecting rich/lean with respective to the theoretical air-fuel ratio, has been generally been performed. However, coping with the recent improvement of exhaust emission characteristic and the increase in demand for improvement of fuel economy, there has been developed a lean combustion engine in which an extremely higher air-fuel ratio (for example, 20-40) compared to the theoretical air-fuel ratio is set to be a target air-fuel ratio. Thus, a wide range air-fuel ratio sensor capable of detecting a wide air-fuel ratio region as also an oxygen sensor, has been utilized.
The air-fuel ratio sensor mentioned above comprises an oxygen concentration detection section consisting of a solid electrolyte for outputting a detection signal corresponding to an oxygen concentration within a hollow chamber to which an exhaust gas from the internal combustion engine is introduced, an oxygen pump section for controlling an electric current to be applied to a solid electrolyte wall that divides the hollow chamber from the exhaust side of the engine, to flow oxygen into/out of the hollow chamber, so that the oxygen concentration in the hollow chamber becomes a predetermined oxygen concentration.
In such a wide range air-fuel ratio detection apparatus, there are some apparatuses that correct output dispersion (inactivity and the like) due to temperature characteristics of the air-fuel ratio sensor (refer to Japanese Unexamined Patent Publication Nos. 60-27751, 1-301939).
However, conventionally, there has not been performed such a correction to dispersion at the manufacture time of circuit for detecting an air-fuel ratio or dispersion of temperature characteristics corresponding to electric current applied to the solid electrolyte (to be referred to pump current hereinafter).
The present invention has been achieved by paying attention to such conventional problems and aims at correcting dispersion of a detection circuit for detecting an air-fuel ratio corresponding to a pump current of a wide range air-fuel ratio sensor (sensor element), to thereby maintain detection accuracy to a high level.
Another object of the present invention is to perform the correction of the air-fuel ratio when such a correction does not affect an engine operation.
A further object of the present invention is to perform the correction of the air-fuel ratio at high accuracy without any influence by a noise or a transitional variation.
For achieving the above objects, the present invention is constructed as follows.
An oxygen concentration detection section of a sensor element outputs a detection signal corresponding to an oxygen concentration in a hollow chamber to which an exhaust from an internal combustion engine is introduced, an oxygen pump section controls an electric current to be applied to a solid electrolyte wall that divides the hollow chamber from an exhaust side of the engine, to flow oxygen into/out of the hollow chamber, so that the oxygen concentration in the hollow chamber becomes a predetermined oxygen concentration.
An air-fuel ratio detection circuit outputs an air-fuel ratio detection value based on the electric current applied to the solid electrolyte wall by the oxygen pump section.
With the above construction, a power supply to the solid electrolyte wall by the oxygen pump section is cut off by a pump current cut off circuit while detecting an air-fuel ratio over a wide range, and the air-fuel ratio detection value is corrected based on an output value from the air-fuel ratio detection circuit at this time.
According to such a construction, when a normal air-fuel ratio sensor is used, the electric current to be applied to the solid electrolyte wall is controlled by the oxygen pump section so that the oxygen concentration in the hollow chamber becomes an oxygen concentration corresponding to a predetermined air-fuel ratio (typically, a theoretical air-fuel ratio). The direction of applied current is reversed depending on the richer/leaner of an exhaust air-fuel ratio to the predetermined air-fuel ratio. If the exhaust air-fuel ratio is equal to the predetermined air-fuel ratio, the electric current flowing to the solid electrolyte wall corresponds to 0.
Accordingly, when the power supply by the oxygen pump section to the solid electrolyte wall is cut off by the pump current cut off circuit, the air-fuel ratio detected by the detection circuit is to be the predetermined air-fuel ratio.
Therefore, when the power supply to the solid electrolyte wall is cut off, based on a deviation between an output value of the detection circuit and an output value corresponding to the predetermined air-fuel ratio, a correction can be performed on a detection value of air-fuel ratio detected over a wide range at the normal usage time when the pump current of the air-fuel ratio sensor is supplied.
Thus, detection accuracy of air-fuel ratio can be maintained to a high level.
Further, the construction may be such that the air-fuel ratio detection value is corrected under an operation condition that an engine air-fuel ratio feedback control based on the air-fuel ratio detection apparatus is stopped.
According to the above construction, since a detection result of the air-fuel ratio detection apparatus is unnecessary under the operation condition that the engine air-fuel ratio feedback control is stopped, the air-fuel ratio detection value is corrected under such an operation condition, resulting in no influence to the engine operation.
Moreover, the operation condition that the air-fuel ratio feedback control is stopped includes the inactivity of sensor element, so that the correction of detection circuit can be performed under a condition that the exhaust air-fuel ratio cannot be detected.
In the above, the air-fuel ratio detection value may be corrected at the time when a fuel supply to the engine is stopped.
In this way, even when the fuel supply to the engine is stopped at the deceleration time and the like, the air-fuel ratio feedback control is not performed and the detection result of the air-fuel ratio detection apparatus is unnecessary, therefore, the correction of the air-fuel ratio detection value during this time does not affect the engine operation.
Moreover, the air-fuel ratio detection value may be corrected, based on a value obtained by averaging process of the output value of the air-fuel ratio detection circuit of when the power supply to the solid electrolyte wall by the oxygen pump section is cut off.
In this way, by average processing the output value of the detection circuit, the air-fuel ratio detection value can be corrected at high accuracy based on an output value avoiding any influence by a noise or a transitional variation.
Furthermore, a correction value of the air-fuel ratio detection value just before the engine operation stop may be back-up stored to use an initial value for the next operation.
In this way, the correction value of the air-fuel ratio detection value most newly updated during engine operation is used as an initial value when the air-fuel ratio detection value at the next operation is corrected, to thereby ensure a high accurate air-fuel ratio detection from the beginning of engine operation start.
Furthermore, the detection circuit may consists of a current/voltage conversion circuit for converting the supply current to the solid electrolyte wall by the oxygen pump section to a voltage signal and an A/D conversion circuit for converting an output value from the current/voltage conversion circuit to an air-fuel ratio detection value. With this construction, the air-fuel ratio detection value is corrected by comparing the output value of the current/voltage conversion circuit at the time when the power supply to the solid electrolyte wall by the oxygen pump section or the air-fuel ratio detection value converted by the A/D conversion circuit with corresponding reference value.
According to this construction, the detection of air-fuel ratio is performed by converting the supply current to the solid electrolyte wall by the oxygen pump section to the voltage signal by the current/voltage conversion circuit and converting this analog voltage to a digital air-fuel ratio detection value by the A/D conversion circuit utilizing an AND conversion table.
Further, the air-fuel ratio detection value may be corrected by correcting the output value (voltage) of the current/voltage conversion circuit such that the output value and/or the conversion table is corrected so that a deviation between the output value of the current/voltage conversion circuit of when the pump current is cut off and an output value corresponding to the abovementioned predetermined air-fuel ratio becomes smaller. Or, the air-fuel ratio detection value may be corrected by correcting the current/voltage conversion circuit of when the pump current is cut off such that the air-fuel ratio detection value (digital value) of the A/D conversion circuit is corrected so that a deviation between the air-fuel ratio detection value converted by the A/D conversion circuit and the predetermined air-fuel ratio becomes smaller.